Multiple carriers exist at present, which provide radio communication systems employing different communication schemes (for example, cdma2000 and W-CDMA). A frequency bandwidth that can be assigned to the carriers is determined in advance, and each carrier is assigned a frequency used in the radio communication system from the limited frequency bandwidth.
Such a radio communication system is a radio communication system that provides real-time communication, and thus has a wide service area to avoid interruption of the real-time communication.
Since each radio communication system (a first radio communication system and a second radio communication system) has the wide service area as mentioned above, the service areas (regions) of the first radio communication system and the second radio communication system usually overlap each other.
Since the frequency used in each radio communication system (the first radio communication system and the second radio communication system) is assigned from the limited frequency bandwidth, it is assumed that the frequency used in the first radio communication system and the frequency used in the second radio communication system may overlap, or may be adjacent to each other.
In such a case, the communication performed between a second mobile station capable of communicating with the second radio communication system and a second base station included in the second radio communication system may interfere with a first mobile station capable of communicating with the first radio communication system.
When the second radio communication system, interferes with the first mobile station as mentioned above, the following method has been proposed as a method of avoiding interference from the second radio communication system.
Specifically, when a ratio of a reception power of a signal received from the first base station included in the first radio communication system and a reception power of a signal received from the second base station is not less than a predetermined threshold, the first mobile station performs handoff processing to switch the frequency used to communicate with the first base station to a frequency not interfered with by the second radio communication system (for example, Japanese Patent Application Publication No. 2003-18641 (claim 1, FIG. 7, and others)).